1. Field of the Invention
This invention relates to cutting of crystalline materials such as quartz, silicon and gallium arsenide and, more particularly, to slicing ingots of such materials into thin wafers.
2. Prior Art
Integrated circuits are typically manufactured on a substrate of monocrystalline quartz, silicon, gallium arsenide, or other semiconducting material. Fabricating the substrate presents unique manufacturing challenges since the material is hard and brittle and must be sliced into very thin wafers. Numerous devices and techniques have been developed for performing this difficult process.
One such technique employs a thin, circular, high-strength steel blade having a central hole whose circumference is coated with an abrasive, such as diamond dust. A semiconductor ingot is positioned within the central hole of the blade and the the blade is rotated. The ingot is then forced against the rotating cutting surface of the blade. Proper tensioning of the blade with this technique is quite difficult and the blade is subject to an unacceptable amount of wander. Vibrations of the blade tend to produce a surface finish on the wafers which requires a subsequent polishing operation.
Another technique employs a gang of circular blades, each having its outer circumference coated with an abrasive material. The blades are rotated against a semiconductor ingot which is also rotated. With this technique, the resulting wafers are left with a fracture defect at their centers. These defects must then be removed by a subsequent grinding operation.
Yet another technique employs a gang of high strength, typically steel, flat blades or wires which are coated with an abrasive material. A wafering machine for performing this technique is described in U.S. Pat. No. 3,272,195 entitled "Device for Slicing Crystalline Material" issued Sept. 13, 1966 to J. H. Myer et al. Assemblies of flat blades adapatable for use in such a device are described in U.S. Pat. No. 3,168,087 entitled "Wafering Machine" issued Feb. 2, 1986 to R. W. Anderson and in U.S. Pat. No. 3,263,669 entitled "Wafering Machine" issued Jan. 30, 1963 to W. H. Ashley. U.S. Pat. No. 4,187,828 entitled "Cutting" issued Feb. 12, 1980 to Schmid describes a technique for making flat steel or wire blades with an imbedded abrasive on only the leading cutting edge, thereby reducing kerf loss. A cutting element comprising a monofilament polymer charged with abrasive particles adaptable for use with this same general techinque is described in U.S. Pat. No. 4,172,440 entitled "Cutting Monofilament" issued Oct. 30, 1979 to Schneider el al.
Each of the above-mentioned references relates to a technique in which a semiconducter ingot is rigidly held and plunged into the gang of blades or wires as they are reciprocated in a direction perpendicular to the axis of the ingot. Maintaining proper tensioning of the individual blades or wires is difficult. Furthermore, the blades or wires are frequently prone to uneven abrasive impregnation and premature abrasive pullout.
A similar technique employs a gang of high strength, flat steel blades which are not coated with an abrasive. Blade assemblies for use with this technique are described in U.S. Pat. No. 4,092,971 entitled "Self-Contained, Multi-Blade Package for Slurry Saws and the Like" issued June 6, 1978 to Viscount and in U.S. Pat. No. 4,387,698 entitled "Slurry Saw Blade Head Assembly" issued June 14, 1983 to Bustany. A slurry of abrasive particles suspended in water or oil is introduced between a semiconductor ingot and the blades. As in the previous technique, the gang of blades is reciprocated in a direction perpendicular to the axis of the ingot. This technique results in relatively uneven cutting action which causes a poor surface finish on the wafers. As with the blades or wires used in the previously described technique, it is difficult to achieve proper tensioning and spacing of the blades.
A more recently developed technique employs a laser beam to cut through a rigidly held semiconductor ingot. This technique requires an exceptionally large capital investment and provides a relatively low rate of production. Furthermore, the heat of the cutting action tends to damage the crystallinity of the semiconductor wafers.